PROJECT SUMMARY A tremendous effort has been undertaken to understand the pathophysiology underlying addiction. In 2017, an estimated 19.7 million people within the United States met criteria for Substance Use Disorder. Disruptions in several interconnected brain regions manifest as behaviors used to diagnose addiction disorders, which includes compulsive drug-seeking as a hallmark symptom. Compulsive behaviors represent a state of impaired control over repetitive actions or thoughts that can be deleterious to normal behavioral function. There is currently no treatment that addresses this symptom in addiction. Implicated in compulsive behavior pathophysiology are disruptions in nucleus accumbens, a central node critical in driving motivated behaviors. These disruptions are specifically associated with excessive activity of medium spiny projection neurons (MSNs) that express the Drd1 dopamine receptor (D1-MSNs), relative to those that express the Drd2 dopamine receptor (D2-MSNs). MSNs comprise the majority of cells within the nucleus accumbens, and strengthening of excitatory synaptic input onto D1-MSNs contributes to addiction-related behavior. This proposal aims to ultimately restore nucleus accumbens function by indirectly engaging a mechanism of synaptic plasticity that will selectively reduce D1-MSN activity. Our preliminary studies inhibited the peptidase activity of angiotensin converting enzyme (ACE), which is highly expressed by D1-MSNs but not by other neuronal cell types or brain regions. ACE has been shown to degrade endogenous opioid peptides released by D2-MSNs called enkephalins. Preliminary findings suggest a unique mechanism where ACE constrains opioid-mediated regulation of D1-MSN function but not D2-MSN function, and ACE inhibition decreases excitatory synaptic input onto D1-MSNs. Thus, we hypothesize that ACE inhibition elevates endogenous levels of enkephalins which activate presynaptic opioid receptors on excitatory terminals, thereby depressing synaptic input selectively onto D1-MSNs. We propose two aims that will 1) directly evaluate the contribution of endogenous enkephalins in regulating D1-MSNs activity and 2) investigate the mechanism underlying this phenomenon mediated by ACE inhibition. The first aim will integrate optogenetics with peptide quantification techniques and whole-cell patch clamp electrophysiology to directly measure the physiological impact of endogenous enkephalins. The second aim will utilize electrophysiology and genetic manipulations to identify the pre- and postsynaptic elements that underly synaptic changes mediated by ACE inhibition. This proposal?s incorporation of interdisciplinary techniques at the intersection of basic science mechanisms and translational application will provide unparalleled training potential towards becoming an independent physician scientist.